# Load Cells Tell You To Lay Off The Donuts

Our old algebra teacher used to say, “You have to take what you know and use it to get what you don’t know.” That saying always reminds of us sensors that convert physical quantities into things our microcontrollers can measure. Sometimes the key to a project is knowing what kind of sensor will read the physical properties of the system you are interested in. If that physical property is weight, you can use what is known as a load cell. [DegrawSt] uses four 50 kg load cells to create a bathroom scale using an Arduino.

Load cells typically contain strain gauges that change resistance when deformed. This actually measures force, but if you mount them so they measure the force exerted by you standing on a platform, you get a scale. A load cell usually has four strain gauges in a bridge configuration. This causes a voltage across the bridge, although the output can be noisy and on the order of millivolts.

There are other types of load cells that use a piezoelectric material, hydraulics, pneumatics, or other technologies. However, the strain gauge cell is the most common. If you want more information about load cells, check out [Rick Sellens’] lecture on the topic, below.

To provide excitation to the load cells and measure the voltage output, you usually want to use an amplifier to condition the circuit. [DegrawSt] uses an HX711 chip on a breakout board to manage the cells. There’s a library for the Arduino already available and even some example code.

The four load cells allow the 50 kg sensors to read a person’s weight, up to 200 kg, anyway. The load cells themselves are in a bridge configuration which adds the weight on each cell together.

If you want to peek inside a commercial scale, we’ve seen that before. If you don’t care about watching your figure, perhaps you’d rather tension your bandsaw.

## 22 thoughts on “Load Cells Tell You To Lay Off The Donuts”

1. Ostracus says:

My load cells say “I don’t register anything, are you eating right”?

2. brian says:

“…person’s weight, up to 200 kg…” nice margin, author for sure is from U.S. ;)

1. Whatnot says:

My impression is that obesity is growing worldwide, so it makes it future-proof too.
Also, sometimes people hold their pet to weigh it and subtract it from their own weight, and some pets are not following their diet. And talking of which, with this design you could make an elongated scale to measure your pet’s weight directly without a trip to the vet.

3. Ren says:

My Craftsman band saw has a built in strain gauge to measure band tension, but it is powered by some goofy camera battery that needs replacing every couple years.

1. Reg says:

Set it to the proper tension and then try to memorize the pitch of the blade. A couple years practice and you should have it down. Make sure you keep the guide positions the same. I’ve set the tension on mine by pitch ever since I bought it, but I never had a means of measuring tension.

1. Since pitch could mean other things in the blade world, I’ll clarify for the rest of us. He means the note plucked on it like a guitar string. Being able to pluck the same note on the same size blade = same tension as before. This is a pretty accurate way to do it. A similar method can be used for setting equal tension on 3D printer belts.

1. Dax says:

Buy a guitar tuner, or use a cellphone app and write down what note you’re playing.

2. TGT says:

If you felt like it you could always measure the voltage of that battery, get a matching regulator and a rectifier, then run it off the same power supply that drives the motor.

1. David says:

Or a solar panel, diode and supercap. I’ve been planning on doing this but not got round to it.

3. Dylan says:

A full size power tool, powered off of mains, that has an internal component powered by a battery?!?! As I write this I realize that I have a Craftsman circular saw, powered by a 19.2V battery pack. It has a laser site powered by 1 or 2 AA batteries. Quite silly to have AA batteries in a device that has a huge (comparatively) lithium battery a mere 4″ away.

How much more could it have cost to include a component to drop the voltage down from the main battery pack to power the lower voltage accessory? The answer, I guess, is “more than the two tabs of metal for the AA battery contacts”

Back to your band saw- it should be pretty easy to just find a small power supply with output voltage matching the battery, and wire that into the AC lines & battery contacts.

I wonder if this was a workaround to a bigger technical problem though. Perhaps the load cell signal amplifier requires very clean DC power and Craftsman couldnt find a cheap enough linear power supply, and said “F it, just put in a battery. Here’s a camera battery that matches our oddball required input voltage, we’ll use that”

1. I think that’s so you can physically disconnect the power source to the motor so it cannot accidentally turn on while you’re setting things up.

4. Whatnot says:

1. Ren says:

They are getting harder to find…

1. Whatnot says:

Maybe you can fashion another battery holder for a more common battery with the same voltage.

4. Nikki says:

You cannot guarantee that 4 50Kg loadcells will survive ‘up to 200Kg’ because the reason you have to sum all 4 is that load isn’t necessarily equally spread across the 4 corners of the scale, if you bottom out one sensor before the rest, it’s possible to damage that sensor long before a 200Kg load is present.

1. Dylan says:

Its also very feasible to install hardstops to prevent load cell damage. I worked on a project where we used a load cell and supporting circuitry from an off the shelf food scale, but added a steel stand-off to prevent deflection past a designated point. In this article’s application, you would want to monitor the signal from each load cell, and if any of them are at the max load/displacement you allow, trigger an “uneven load” signal to the user. If all load cells are at the max load/displacement, you trigger an “over limit” signal to user.

1. Whatnot says:

Good idea to give feedback, but I’m not sure the travel isn’t so slight that it will work to make the thing operate to 50kg and then stop it above that. Maybe I’m wrong and there is plenty of travel though, no idea. Hell, looking at that load cell it actually might have a stop incorporated in its design already from factory.

2. CityZen says:

Just put a pointy plastic cone on each corner of the scale to make sure no one stands there. :-)

1. Whatnot says:

Now that’s practical.

2. Ren says:

or a LEGO
B^)

5. GTG says:

Avoid the middleman and go straight to Instructables to read articles.

1. Whatnot says:

it’s a sea of articles and not all of them are worth much so HaD picks a few that seem interesting. Plus the link in the HaD writeup goes to instructables for those interested in making their own.
Middlemen exist for a reason, and plenty of times good reason.

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